Our primary objectives are: 1. to perform a comprehensive evaluation of the effects of use and disuse on the physiological properties of fast and slow skeletal muscle, and 2. to correlate the physiological changes with selected biochemical studies on the contractile protein myosin, and the Ca ions regulatory sytem, the sarcoplasmic reticulum (SR). Perhaps the most interesting finding of our current studies is that disuse (decreased motor activity) affects all three fiber types in specific, and often different, ways. For example, disuse shortens the isometric twitch duration in the slow soleus while prolonging this contractile property in fast muscle. We have found the isometric contractile properties to change rapidly in response to disuse reaching new steady-state levels within seven days. In contrast, the maximal isotonic shortening velocity (Vmax) does not undergo a significicant change until at least four weeks of disuse. During recovery from disuse, all of the contractile properties return rapidly to control levels. However, muscle mass is still depressed after 56 days of recovery. We have observed disuse to stimulate both myosin ATPase and the maximal isotonic shortening velocity (Vmax). The correlation of these parameters gives added support to the concept that Vmax is limited by the rate at which myosin can split ATP. Our data also suggest that the rate of tension development during an isometric twitch is controlled by the SR activity and not by myosin. These results are important for they link particular subcellular protein systems as control points in the mechanical events of muscle function.